1. Related Applications
The present application is related in subject matter to the following U.S. patent applications filed simultaneously herewith of: Ser. No. 341,730 filed in the names of Richard C. Rice, Barry N. Stone and Lynn J. Orendorff and entitled SYSTEM AND METHOD FOR CONTROLLING EXPOSURE FORMAT FOR AN APPARATUS FOR EXPOSING PHOTOGRAPHIC FILM WITH IMAGE DATA; and Ser. No. 341,731 filed in the names of Richard C. Rice entitled CALIBRATION APPARATUS FOR A LIGHT SOURCE USED FOR EXPOSING PHOTOGRAPHIC FILM WITH IMAGE DATA.
1. Field of the Invention
The present invention generally relates to apparatuses for exposing photographic film with image data and, in particular, to control of the exposed film density produced by such apparatuses.
2. Statement of the Prior Art
Photographic film has become the accepted archival medium for medical imaging because of its high analog resolution and because of its long time use with x-rays. Conversely, modern medical imaging techniques, such as computerized tomography, digital radiographic imaging, ultrasound and magnetic resonance imaging, have developed around the use of digital imaging techniques where an image is made up of a great many pixels each of which has an electronically represented brightness. In order to adapt these new techniques to the accepted archival medium, apparatuses have been developed for creating a hard copy by "printing" the electronic image data onto photographic film. These apparatuses are generally referred to as hard-copy cameras.
Thus far, the two primary approaches have used lasers and cathode ray tubes as light sources to expose the film by illuminating each pixel to the proper intensity. These devices have experienced some problems in controlling the exposure or writing intensity and the resulting film density due to several inherently unstable factors. Exposure intensity is generally affected by the light producing response of the light source, and film density is affected by the exposure intensity, of course, and also by the response of the film and the developing chemicals.
One attempt at controlling the various factors involved is described in U.S. Pat. No. 4,700,058. This patent describes a general scheme for using an exposure intensity calibrated at a predetermined level, for producing a gray scale on a film sample and then measuring the film density against a standard after developing of the film. The measured densities are then used to control an input amplifier, or a look-up table in the digital version, to provide compensation for the response indicated by the measured film densities. The exposure intensity is periodically recalibrated using the same predetermined level to adjust for short term variations in light intensity.
Although this technique takes into account short term intensity variations and film density variations, it does not calibrate the light source in response to film density, and it does not allow for variations of the input data format or compensate for its effect on exposure intensity. Exposure intensity in the prior patent is calibrated to predetermined black and white levels or references located within the instrument. Film density is then measured, and the adjustment data is stored in the data input section by means of either a look-up table or bias and gain settings for an amplifier. Unfortunately, it is possible for the calibrated light intensity levels to differ significantly from the light intensity levels corresponding to the densities at which the film is measured. In other words, the film density may be measured using one set of exposure intensity levels while the exposure intensity is calibrated at a different level. This condition can introduce gray scale inaccuracies due to the nonlinear responses of the light source, the film and the developing chemicals. Further, this prior approach does not allow the instrument to be used with mage data having different formats.